With the advance of science and technology, medical imaging has obtained great development, and more imaging modes have become available, such as X-Ray photography, magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET) and so on, each of which with its strengths may complement each other.
PET is a fairly advanced clinical imaging technique in the field of nuclear medicine. In PET, a positron generated by the decay of a radionuclide gets in collision with a negatron in vivo, then the positron and the negatron are annihilated with each other, two gamma photons are emitted in almost opposite directions. Functional information relating to metabolic activities may be obtained by way of detectable gamma rays to diagnose a disease. A PET image may show the functional information and identify a tumor. However, the resolution of the PET image may be low.
CT is another clinical imaging technique in which a specific part of a body with a certain thickness may be scanned. For instance, when X rays pass through human tissues, a portion of the X rays may be absorbed by the tissues, and a portion passing through the body may be detected by a detector on the basis of which a corresponding signal may be generated. Corresponding to the differences in densities of various tissues and the differences in the x-ray penetration abilities, the detected rays may be different. The signal corresponding to the detected rays may be converted to a digital signal. The digital signal may be processed by a computer, and then an image may be generated and displayed. A minor lesion in vivo may be identified based on the image. The CT may generate an anatomical image with a high resolution and a high sensitivity in identifying the morphology of a tissue. However, CT may lack the capacity to determine other characteristics of the lesion.
PET/CT is a technology combining CT with PET. It may provide the information of CT and PET with only one diagnostic examination using a same table and a same image processing workstation. Fused images may be obtained by image reconstruction and image fusion. The fused images may show both functional and anatomical information. The fused images with more complementary information and higher resolution may improve diagnostic accuracy. For instance, the fused images may provide more information for making a treatment plan for tumor.
A PET/CT fused image may be obtained by superimposing one image on another of a same anatomical location or level. In this case, the vertical position of the table on which a patient may be placed for examination may need to be controlled to facilitate the image fusion.
Usually, a plate of the table supported at only one end may be displaced relative to the CT scanner in an axial direction during a CT scanning. When the plate extends out, because of the weight of a patient placed on the plate, the plate may bend within a scanning cross-section. The scanning cross-section may be a plane perpendicular to the rotation axis and through an iso-center of the CT scanner. The plate may also bend during a PET scanning. The CT image and the PET image at a same anatomical location or level may need to be matched so that the CT image and the PET image may be fused. Hence, the bending of the plate may need to be corrected in order to match a CT image with a corresponding PET image.
Besides, in the PET/CT system as illustrated in FIG. 1(a), a table unit 13 includes a base 133, a bracket 132 and a plate 131. Plate 131 may move relatively to the bracket 132 along the axial direction. There is a position 1331 for a CT scan (shown as a dotted line in FIG. 1(a)) and a position 1332 for a PET scan (shown as a dotted line in FIG. 1(a)) on the base 133 along the z axis parallel to the rotation axis of the CT scanner 11 (i.e., the length direction of the plate 131). When the bracket 132 is placed at position 1331 for a CT scan, a patient may be placed onto or removed from the table unit 13 (e.g., the plate 131 of the table unit 13). The plate 131 may exit from the bore of CT scanner 11 completely to avoid collision with CT scanner 11 when the plate 131 is moved up or down. In this case, the distance between one end of the plate 131 close to CT scanner 11 and the scanning cross-section is large. CT scan may not be performed before the plate 131 arrives at the scanning cross-section. In the case of a fixed maximum moving distance of the plate 131, the further an initial position of the plate 131 is away from the scanning cross-section, the smaller the scanning range of CT scanner 11 is.